I do realize that this is an internet forum full of morons, however in real life, no one questions me, people look to me for the answer, look up to me, trust me. To have dipshits on a video game forum question me, is insulting.

Vanilla - can't comment, didn't play during it.
TBC - can't really comment, as didn't play during it, but if I judge Outland and the raids then...7/10
WotLK - 8/10
Cata - 6.5/10 - only reason it's not around 5/10 is because I really enjoyed the revamped EK and Kalimdor.
MoP - so far, 8.5/10

I'm not really a big fan of just giving a score and I go OCD with it and need to add a reason, sorry. Also I rate them for the times, as well as I can.

Vanilla 9/10 - reason: it was rather innovativ and did a lot of new things (yes they ripped others off, but looking at the entire game, it was different)
TBC 7/10 - Still fresh and added a lot of new things and changed a lot of things that where downright aweful.
WoTLK 7/10 - Same as above.
Cata 4/10 - added some things that where good, but also made some design and gameplay decision that I for one did not like. Also not really a new continent, just new zones on Azeroth.
MoP 6/10 - I rate it rather average since it is what I think of MoP. It is by now a pretty average MMO in my eyes.

vanilla - 6/10 - slightly above average. it was awesome when compared to the other options at that time, but I'm picky, so I'm using nowadays standards for comparison.

BC - 7/10 - as much as I love burning legion lore, as much as the sky was beautiful and as much as I enjoyed the expansion overall, BC lacked much in retrospect, and I'm not even talking about future features. I'm talking about class and spec balance, and stuff for the casual crowd to have fun on.

WotLK - 9/10 - the expansion where I bloomed and ceased to be a noob, and I have LOADS of nostalgia about it, but when subjected to an objective analysis, it wasn't perfect by any means. it was a great improvement from BC though. the lightning fast ulduar and that whole year of ICC are probably the main reasons wrath doesn't get a 10.

cata - 4/10 - cata gets a 4 from me for transmog, LFR, old world revamp, molten front and the last 3 dungeons (especially well of eternity, because I'm an illidan fanboy ), but the core of the expansion was bad. the story was weird (the villain's goal was to kill everybody and than die too, neptulon is still wrestling with ozumat, probably on some hentai website, al'akir joins deathwing for no apparent reason other than the rule of cool, et cetera), there was nothing to do besides logging on twice a week for raiding and the raids themselves were short and unsatisfactory. the spin from ultra hard dungeons to very easy dungeons also felt rather weird, like the game's target audience and design philosophy suddenly changed completely. I liked the easier dungeons better than the old ones, but it kind of felt too sudden.

MoP - 8/10 - mob is brilliant, but not without it's flaws. the game went from "there's nothing to do" to "there's too much to do, I can't go to college!", and leveling alts is now a painful chore. no flight until 90 was nice... until I was leveling my 5th toon. I saw the world already on my first and second playthroughs (from both alliance and horde PoVs), and there's no meaning on remaining stuck on the ground to see it again besides making my life miserable. pet battles and the farm are really nice now,, and it's good to see how accessible the game is becoming. MoP would easily get a 10/10 if it weren't for those issues.